1. Technical Field
The invention relates to a lens block having a structure, which is configured so that a plurality of lens elements for constituting a part or all of an optical system can be handled integrally, a lens holder for holding the lens block, an a projector using the lens holder.
2. Description of the Related Art
There has been known an RPTV (Rear Projection TV) in which a projector unit is placed behind a large screen so as to project light on the screen (JP 2003-114479 A). The RPTV is widely used because it is cheaper in unit price per screen size than any other television such as a plasma TV or a liquid crystal TV. However, the RPTV has a defect in its thickness because the projector unit has to be put behind the screen.
Therefore, there has been proposed a low-profile RPTV in which a projection lens provided in a projector unit is folded substantially like a V-shape so that the length of the projection lens can be prevented from increasing the thickness-direction size (JP 2000-187274 A corresponding to GB 2343966 A).
The projection lens disclosed in JP 2000-187274 A has an optical path converting unit such as a prism, by which the optical axis of image light passing through a first optical lens group constituting a projection lens is bent at a fixed angle, and the image light with the bent optical axis is made incident on a second optical lens group. The first or second optical lens group is constituted by a plurality of lens elements. Particularly, large-diameter lens elements are used in the first lens group. It is therefore troublesome to handle the first or second optical lens group. It is desired to form the first or second optical lens group to have an integrated structure.
In the RPTV, reduction in the cost is required. Therefore, moldings made of plastic are used as many parts. Particularly in a body tube, a lens barrel, a lens frame, etc. for holding a plurality of lenses, prisms, etc., with high precision is required, so that the manufacturing cost increases. Therefore, those parts are often molded out of plastic materials with a mold.
A lens block constituted by a plurality of lens elements is held in a lens barrel in the related art. In the lens barrel, stepped portions are formed in its inner circumferential surface. Each stepped portion has an abutment surface parallel to an optical axis, which surface serves to adjust the center of the corresponding lens element, and an abutment surface perpendicular to the optical axis, which surface serves to position the corresponding lens element along the optical axis. A plurality of such stepped portions are formed in the inner circumference of the lens barrel so as to incorporate the plurality of lens elements. As a result, it is necessary to process or form the lens barrel so precisely that the cost of the lens barrel increases.
After the lens elements are incorporated in the lens barrel, projection resolution is examined or MTF is measured. When adverse influence of misalignment of the optical axis is detected, the lens elements are removed from the lens barrel, assembled and adjusted again.
Therefore, there has been known a lens system assembling method in which lens elements are fixedly bonded before and behind spacer rings (JP Sho.59-68710A). In this assembling method, lens elements which have not been centered are disposed before and behind spacer rings, and their optical axes are adjusted and aligned. Thereafter, an adhesive is applied to gaps between the lens elements and the spacer rings so as to temporarily fix them. A lens block fixed temporarily is inserted into a mold. Insert molding of resin is then performed all over the periphery of the lens block. Thus, a lens barrel made of resin is formed in the outer circumference of the lens block.